Electronic devices such as solar cells, field-effect transistors and OLEDs can be composed of organic materials disposed in stacked layers. Devices of this type may require two separate charge transporting regions or units that must be physically separated from each other while maintaining good electric and physical contact between them so that current can flow efficiently through both regions. This connecting region can be composed of multiple layers with different electronic properties so that current can pass with reduced resistance. The layer(s) that make up the connecting region can have various names such as charge generation layer(s) or recombination layer(s). One common arrangement of this type is known as a ‘pn’ also ‘pin’ or ‘p-i-n’ junction. This is typically composed of a layer with p-type (hole generating) and a layer with n-type (electron generating) properties. In this way, a region with high conductivity is created.
Organic electronic devices with these kinds of internal connecting regions are known. In particular, U.S. Pat. No. 6,717,358 discloses tandem OLEDs with two electroluminescent units where the connecting region is formed from a n-type doped layer, interfacial layer, p-type doped layer arrangement where the interfacial layer must be transparent and formed from a semi-conducting material with bandgap <4.0 eV. Similarly, U.S. Pat. No. 7,273,663 discloses OLEDs with two electroluminescent units where the connecting region is formed from a n-type doped layer, metal compound layer, p-type doped layer arrangement where the metal compound can be a metal oxide, sulfide, selenide or nitride. US2006/0263629 discloses OLEDs with two electroluminescent units separated by an “intermediate electrode” of an electron-injection layer, mixed metal-organic material layer, hole-injecting layer arrangement. The organic material in the mixed layer may include phthalocyanines among other materials.
U.S. Pat. No. 7,494,722 discloses tandem OLEDs where the intermediate connector is formed from a n-doped organic layer and an electron-accepting layer containing a compound with a reduction potential greater than −0.5 eV (vs. SCE). The material in the electron accepting layer may be a hexaazatriphenylene or fluorinated derivatives of tetracyanobenzoquinonedimethane. There may be an interfacial layer between the n-doped organic layer and the electron accepting layer. These same devices have also been disclosed in Liao et al, Advanced Materials, 20(2), 324-329 (2008).
WO2007/071450 discloses electronic devices with p-n junctions where the n-type molecular dopant has a redox potential <1.5 eV and the p-type molecular dopant has a redox potential >0 eV.
Disclosures of electronic devices which use phthalocyanines as a connecting layer between two charge transporting units include EP 1564826 and U.S. Pat. No. 6,337,492.
Additional examples of tandem OLEDs that include at least two electroluminescent units with an intermediate connector include U.S. Pat. No. 6,872,472, US2003/0127967, U.S. Pat. No. 7,075,231, U.S. Pat. No. 7,494,722, WO2007/027441 and US2008/0171226.
Notwithstanding all of these developments, there are continuing needs for improvements. For EL devices that produce light (for example, an OLED) or convert light into current (for example, a solar cell), there remains a need to maintain or increase high luminance or photoelectric conversion efficiencies and provide long lifetimes. For OLEDs, lower device drive voltages and hence lower power consumption as well as good color purity is also desirable.